This invention relates to a process for removing carbon monoxide, hydrocarbons, and nitrogen oxides in oxidizing gas streams such as may be encountered in exhaust from advanced lean-burn automotive engines or diesel engines.
Three-way catalysts (catalysts which simultaneously oxidize carbon monoxide and hydrocarbons while reducing oxides of nitrogen (NO.sub.x) and oxygen) are widely used to purify toxic emissions of automotive internal combustion engines. These catalysts typically contain precious metals, such as platinum, palladium, or rhodium, together with promoters such as ceria supported on alumina. The catalysts function by using carbon monoxide, hydrogen, and hydrocarbons to reduce NO.sub.x when the amount of oxygen and of NO.sub.x present are sufficient but not in significant excess of the amount required to oxidize completely the carbon monoxide, hydrogen, and hydrocarbons in the exhaust gas. Therefore, engines are controlled to operate so that a sufficient but not excessive amount of air is allowed to react with the fuel.
With respect to fuel consumption, it is more economical to operate automobiles powered by internal combustion engines under lean conditions, i.e., with oxygen present in excess of that required to react completely with the fuel during combustion. Unfortunately, the emissions of these so-called lean-burn engines also contain excess oxygen that is substantially in excess of the amount required to completely oxidize carbon monoxide and hydrocarbons coexisting in the emission stream. Therefore, conventional three-way catalysts do not adequately reduce NO.sub.x in lean gas streams.
Clearly, methods are needed to promote the reduction of NO.sub.x produced in lean-burn engines. Considerable work has been done recently using alumino-silicate zeolites containing copper. But catalysts which contain copper as the only effective component show inferior performance. Their reduction of NO.sub.x increases rapidly with temperature, passes through a maximum, and then declines.
Copper zeolite catalysts that also contain silver are known, but for the decomposition rather than for the reduction of NO.sub.x. (e.g., Kagawa et al. Chemistr Letters, The Chemical Society of Japan, 407-410 (1991), and Austral. Pat. Appl. 722699 (Kagawa et al.)). The decomposition of NO.sub.x, however, is severely inhibited by oxygen and H.sub.2 O, and made substantially impossible by sulfur dioxide, all of which are normal constituents in automotive exhaust. Moreover, the decomposition of NO.sub.x has not been demonstrated in such systems when the inlet concentrations of NO.sub.x are below about 1500 ppm, as is the case in automotive exhaust.